PostScript monthly FAQ v2.4 07-18-95 [11 of 11]
Allen Braunsdorf
Answers to Questions
(the comp.lang.postscript FAQ v2.4)
Allen Braunsdorf
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Now that there is Linux and NetBSD and BSD 386 UNIX IBM PC (and
clone) users can run any of the X-windows and UNIX programs in the
utilities section. See any of comp.os.linux.{
admin,development,help,misc}. Also, there is now GhostScript for
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Many thanks to Dan Carrigan for reformatting the books and
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The utilities index from the comp.sources.postscript FAQ will be
posted in comp.lang.postscript now too.
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Books and programs are referred to by name only. See the book
sections for book information, and the comp.sources.postscript FAQ
for a full list of all PostScript related programs. I have archived
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wilma.cs.brown.edu:pub/postscript. You can get the
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Related FAQs: comp.text, comp.text.tex, comp.fonts, comp.graphics,
comp.sys.mac.apps, comp.sources.postscript.
11 About PostScript 2
11.1 What printers support Level 2 PostScript?
* Apple LaserWriter IIf
* Apple LaserWriter IIg
* Apple LaserWriter Pro 600 (with ram upgrade to get 600 DPI)
* Apple LaserWriter Pro 630 (True 600 DPI)
* Apple Personal LaserWriter NTR
Apple sells an upgrade to the IINTX to turn it into a IIf/IIG for
instance.
* Compaq PAGEMARQ 20
* Compaq PAGEMARQ 15
* Data Products LZR 960
* Data Products LZR 1560
* DEClaser 1152
* Hewlett-Packard PostScript CartridgePlus, which works with the HP
Laserjet III, IIID, and IIIP.
* Hewlett-Packard LaserJet 4M
* NEC SilentWriter 95
* QMS 1725 Print System
* QMS 860 ``Hammerhead''
* QMS ColorScript 210 and 230
* Tektronix Phaser III PXi
* Tektronix Phaser II (all models)
* Tektronix Phase 200e
* Texas Instruments microLaser Turbo
* Texas Instruments microLaser XL Turbo
This rest of file contains a description of PostScript 2 written by
Carl Orthlieb from Adobe. The text has not been changed, but some
paragraphs have been deleted for brevity. Comments by me are in
square brackets, and these were not written by Adobe.
11.2 What is PostScript Level 2?
Since its introduction in 1985, the PostScript language has been
considerably extended for greater programming power, efficiency,
and flexibility.
Typically, these language extensions have been designed to adapt
the PostScript language to new imaging technologies or system
environments. While these extensions have introduced new
functionality and flexibility to the language, the basic imaging
model remains unchanged.
PostScript Level 2 integrates the original PostScript language, all
previous language extensions, and new language features into the
core PostScript language imaging model.
11.3 [ Color Extensions ]
The color extensions were added to the language in 1988 to provide
more complete color functionality. With the original PostScript
language, color could be specified using the red-green-blue (RGB)
and hue-saturation-brightness (HSB) color models.
The color extensions include cyan-magenta-yellow-black (CMYK) color
model, black generation and undercolor removal functions, screen
and transfer functions for four separate color components, and a
colorimage operator for rendering color sampled images. The color
extensions are currently found in PostScript color printers from
Canon, QMS, Oce, and NEC as well as all implementations of the
Display PostScript system.
11.4 [ Composite Font Extensions ]
The composite font technology is a general solution that extends
the basic PostScript language font mechanism to enable the encoding
of very large character sets and handle non-horizontal writing
modes.
A Type 1 PostScript font has room for encoding only 256 distinct
characters. A typical Japanese font has over 7,000 Kanji, katakana
and hiragana characters. The composite font technology allows you
to create one ``composite'' font that is made up from any number of
``base'' fonts. In addition, the composite font technology allows
you to include two sets of metrics (character spacing details) in
the font: one for a horizontal-writing mode, and one for a
vertical-writing mode.
11.5 [ Display PostScript Extensions ]
The Display PostScript extensions address the needs of using the
PostScript language imaging model in a display environment. It
includes extensions to deal specifically with displays and
windowing systems as well as many optimized operators to increase
performance which is critical in an interactive display environment
[ (and printers) ] .
11.6 [ Overview of Level 2 Features ]
11.7 Filters
* A filter transforms data as it is being read from or written to a
file. The language supports filters for ASCII encoding of binary
data, compression and decompression, and embedded subfiles.
Properly used, these filters reduce the storage and transmission
cost of page descriptions, especially ones containing sampled
images. Benefits: Reduced storage requirements, greater
performance.
* ASCII encoding of binary data: ASCII/85 (represent binary data in
ASCII format with only a 125 % expansion of data), and ASCII/HEX
(current method of representing binary data in ASCII format but
with a 200 % expansion of data). Benefits: Compact representation
of binary data in a portable ASCII representation.
* Compression and decompression filters: CCITT Group 3 & 4
(monochrome images), run-length encoding (monochrome and
grayscale images), LZW ( 2:1 compression of text files), DCT
(20-200:1 compression of color images using the proposed JPEG
standard). Benefits: Improved performance due to reduced
transmission times. PostScript files on disk can also be made
much smaller, saving disk space.
11.8 Binary Encoding
In addition to the standard ASCII encoding, the language syntax
includes two binary-encoded representations. These binary encodings
improve efficiency of generation, representation, and
interpretation. However, they are less portable than the ASCII
encoding and are suitable for use only in controlled environments.
Benefits: performance, compactness.
11.9 Optimized graphics operators
* Rectangle operators. New operators for filling, clipping and
stroking rectangles; all highly optimized. For example, rectfill
is 3 times faster than an equivalent moveto, lineto, lineto,
lineto, closepath, fill. Benefits: performance and convenience.
* Graphics state objects provide a fast way to switch between
graphics states, which define the current line weight, color,
font, etc. In existing printers, graphics states are stored on a
stack, so accessing an arbitrary graphics state is somewhat
cumbersome. With graphics state objects, the graphics state can
be associated with a name, and retrieved by simply requesting the
name. Benefits: Performance, convenience.
* Halftone specification. New halftone dictionaries provide a more
precise way of specifying the halftone dots, and makes switching
between halftone screens faster. (The spot function is not
reinterpreted.) Benefits: Performance, convenience, enhanced
functionality.
* User paths are self-contained procedures that consists entirely
of path construction operators and their coordinate operands.
User path operators perform path construction and painting as a
single operation; this is both convenient and efficient. There is
a user path cache to optimize interpretation of user paths that
are invoked repeatedly. Benefits: Performance, convenience.
* Stroke adjustment. For very thin lines, there is a trade-off
between perfect positioning and consistent line width. Depending
on the placement of such a line, it could end up being rendered
as either 1 or 2 pixels wide, which is a noticeable difference.
To account for this, PostScript language programs often include
logic to slightly alter the coordinates of lines for consistent
rendering. With automatic stroke adjustment the interpreter
performs this adjustment to ensure consistent widths. Doing it in
the interpreter rather than in the PostScript language program is
20 - 30 % faster. Benefits: Performance, convenience, improved
quality.
11.10 Optimized text operators
* The xyshow operator provides a more natural way for applications
to deal with individual character positioning. Allows
simultaneous track kerning, pair kerning, and justification.
Benefits: Performance, convenience.
* The selectfont operator optimizes switching between fonts. It
does the work of 3 Level 1 operators: findfont, scalefont, and
setfont and has been optimized by using a caching mechanism.
Benefits: Performance, convenience.
11.11 Forms
* A form is a self-contained description of any arbitrary graphics,
text, and sampled images that are to be painted multiple times on
each of several pages or several times at different locations on
a single page.
* With the new forms feature, you can define a base form whose
representation stays cached between pages, so only information
that changes between forms will need to be interpreted for each
page. The representation used to cache the form may vary from
device to device depending on the available resources, such as
memory and/or hard disk space. In some cases, the actual
rasterized form will be saved, in other cases, an intermediate
representation (such as a display list) may be saved. Benefits:
End-users will benefit by improved performance.
* This makes forms processing faster and provide a natural
framework for ISVs implementing a forms functionality in their
application. Benefits: Convenience for ISVs.
* Besides the traditional concept of ``forms,'' some other examples
of forms include: Letterhead, stationary, overhead presentation
backgrounds, repetitive symbols in a CAD drawing such as screws
(mechanical drawing) or windows (architectural drawing), complex
background blends in 35mm slides. Benefits: Enhanced
functionality and application of PostScript printers in a variety
of different environments.
11.12 Patterns
* The new pattern color space provides the ability to establish a
pattern as the current color. Subsequent use of operators such as
fill, stroke, and show apply ``paint'' that is produced by
replicating (or tiling) a small graphical figure called a pattern
cell at fixed intervals in x and y to cover the areas to be
painted. The appearance of a pattern cell is defined by a
PostScript language procedure, which can include any arbitrary
graphics, text, and sampled images. The shape of the pattern cell
need not be rectangular, and the spacing of tiles can differ from
the size of the pattern cell. Benefits: Enhanced functionality,
performance, convenience.
* For efficiency, the representation of the pattern cell may be
cached. When cached, the execution of the procedure that defines
the pattern need be done only once for the current pattern. The
pattern cache is similar to the font cache. Benefits:
Performance.
* Multiple colors can be specified in the pattern or the pattern
can be used as a mask to paint a color defined in some other
color space. Benefits: Enhanced functionality
* For display environments, this feature will allow patterns to be
represented in a resolution independent manner. Until now,
patterns have typically been represented by arrangements of
pixels. This resolution-dependent representation does not work
well when trying to image the pattern at a variety of different
resolutions.
11.13 Images
There are several enhancements to the facilities for painting
sampled images: use of any color space, 12-bit component values,
direct use of files as data sources, and additional decoding and
rendering options. Benefits: Convenience, performance, quality.
11.14 Composite Fonts
* Provides the basic machinery for non-Roman character sets.
Enables the encoding of very large character sets and
non-horizontal writing modes. Benefits: Enhanced functionality.
* Provides a page description language for international business.
Composite font technology makes printers more international. The
same font technology can be used worldwide, and will provide
support for companies that must work in today's international
business environment. Benefits: Enhanced functionality.
* Advantages not limited to foreign languages - also useful for
strictly Roman printers: allows the creation of a single
composite font that combines two or more fonts. For example, you
may wish to combine a textual font (such as Times-Roman) with a
graphical font (such as Zapf-Dingbats), and have all characters
at their disposal within a single font. Other uses of composite
fonts: IBM extended character set, and expert sets (such as Adobe
Garamond). Benefits: Enhanced functionality and increased
performance by minimizing switching between fonts.
11.15 New Color Spaces
* CMYK color model and support for color images. Enhanced
functionality. This will encourage more ISVs to use the color
operators, because the operators will be widely available (The
printer itself may not be able to print in color, but the
PostScript language program won't generate errors when the
operators for CMYK color are used.)
* PostScript Level 2 supports several device-independent color
spaces based on the CIE 1931 (XYZ)-space. CIE-based color
specification enables a page description to specify color in a
way that is related to human visual perception. The goal of the
CIE standard is that a given CIE-based color specification should
produce consistent results on different color output devices,
independent of variations in marking technology, ink colorants,
or screen phosphors. True device-independent color specification.
Improved color matching between devices.
* PostScript Level 2 supports three classes of color spaces: device
independent, special, and device dependent.
The following device independent color spaces are standard:
The CIEBasedABC color space is defined in terms of a two-stage,
non- linear transformation of the CIE 1931 (XYZ)-space. The
formulation of the CIEBasedABC color space models a simple zone
theory of color vision, consisting of a non-linear trichromatic
first stage combined with a non-linear opponent color second stage.
This formulation allows colors to be digitized with minimum loss of
fidelity; this is important in sample images.
Special cases of CIEBasedABC include a variety of interesting and
useful color spaces, such as the CIE 1931 (XYZ)-space, a class of
calibrated RGB spaces, a class of opponent color spaces such as the
CIE 1976 (L*a*b*)-space and the NTSC, SECAM, and PAL television
spaces.
The CIEBased A color space is a one-dimensional and usually
achromatic analog of CIEBasedABC.
The following special color spaces are standard:
* The Pattern color space enables painting with a ``color'' defined
as a pattern, a graphical figure used repeatedly to cover the
areas that are to be painted. See the discussion of patterns for
more information.
* The Indexed color space provides a way to map from small integers
to arbitrary colors in a different color space such as a device
independent color space.
* The Separation color space provides control over either the
production of a color separation or the application of a device
colorant, depending on the nature and configuration of the
device.
The following device dependent color spaces are standard:
* The DeviceGray color space is equivalent to the existing
PostScript language's gray color model.
* The DeviceRGB color space is equivalent to the existing
PostScript language's red-green-blue (RGB) color model.
* The DeviceCMYK color space is equivalent to the existing
PostScript language's cyan-magenta-yellow-black (CMYK) color
model.
11.16 New screening/halftoning technology
* Improved algorithms for determining the angles and frequencies
used for halftone screens. The improvements fall into two primary
categories: general improvements, and improvements specific to
color separations.
* General improvements: (1) The new algorithms yield a 10 %
improvement in the speed of the setscreen and image operators;
(2) Earlier version of PostScript software could produce halftone
screens only for certain angle and frequency combinations. Enough
of these combinations were available so that any requested screen
could be fairly well approximated by one of the available angle
and frequency combinations. In contrast, the improved halftoning
algorithms can provide as much as a ten-fold increase in the
number of angle-frequency combinations that are available,
depending on the device resolution and the available memory.
Benefits: Increased performance and higher quality halftone
screens.
* Improvements specific to color separations: An additional feature
is available that enables PostScript software to generate
extremely accurate screen angles and frequencies. The screens
produced by this method can achieve an angular accuracy of within
05 degrees or better, depending on such parameters as exact
screen angle requested, device resolution, and memory available
for use by the algorithm. Benefits: Extremely high-quality color
separations that approach the quality that previously was
available only from high-end, color electronic pre-press systems.
11.17 Improved printer support features
* Page device setup provides a device independent framework for
specifying the requirements of a page description and for
controlling both standard features, such as the number of copies,
and optional features, such as duplex printing, paper trays,
paper sizes, and other peripheral features.
* Applications developers will be able to write a single driver for
a variety of different PostScript printers. The same code can be
used to address printer specific features whether the features
exist in the printer or not. If the feature is not in the
printer, the application can decide how to best respond to the
lack of the feature. Benefits: Enhanced functionality. ISVs
benefit by having a more uniform method for accessing printer
specific features. End users benefit by having software that will
take advantage of their printer's features.
11.18 Interpreter parameters
Administrative operations, such as system configuration and
changing input-output device parameters, are now organized in a
more systematic way. Allocation of memory and other resources for
specific purposes is under software control. For example, there are
parameters controlling the maximum amount of memory to be used for
VM, font cache, pattern cache, and halftone screens. Benefits:
Flexibility.
11.19 Resources
* A resource is a collection of named objects that either reside in
VM or can be located and brought into VM on demand. There are
separate categories of resources with independent name spaces -
for example, fonts and forms are distinct resource categories.
* The language includes convenient facilities for locating and
managing resources.
11.20 Dictionaries
Many Level 2 operators expect a dictionary operand that contains
key-value pairs specifying parameters to the operator. Language
features controlled in this way include halftones, images, forms,
patterns, and device setup. This organization allows for optional
parameters and future extensibility. For convenience in using such
operators, the PostScript language syntax includes new tokens,
<< and >>,
to construct a dictionary containing the bracketed key-value pairs.
Benefits: Convenience, extensibility.
11.21 When did Level 2 products come available?
The first Level 2 products were available in early 1991. Level 2
printers will soon be much more common.
11.22 Are Level 1 and Level 2 implementations compatible?
[ Mostly, but not fully. The incompatibilities are of sufficient
magnitude that when QMS introduced its first Level 2 compatible
printer, it provided a ``Level 1'' mode for backward compatibility,
for handling jobs which wouldn't run in Level 2. This should not be
taken as too much of a criticism...some of the incom- patibilities
from Level 1 to Level 2 involved fixing design mistakes in Level 1,
or tightening up definitions which had been loose in Level 1 (where
careless application writers had made use of the looseness, writing
not to the language specification but to particular
implementations.) ]
(C) 1990 Adobe Systems Incorporated. All rights reserved.
PostScript, Display PostScript, and Adobe are trademarks of Adobe
Systems Incorporated registered in the U.S. All other product names
are trademarks or registered trademarks of their respective